Physics: how to do, thanks?

算是很基本的問題.

v(t) = v(0) - gt j
v(0) = 1.5(cos30° i + sin30° j)
     = 1.5(√2/2 i + 1/2 j)
     = (3√2/4)i + (3/4)j
s(0) = 1 j
s(t) = s(0) + v(0)t - gt^2/2 j
     = (3√2/4)t i + (1 + 0.75t - 4.9t^2)j

設 t = T 時落地, 則
    1 + 0.75T - 4.9T^2 = 0
因 T > 0, 解得 T = 0.5347 秒.
此時速度為
    v(T) = v(0) - gT j
         = 1.06066 i + 0.75 j - 5.24027 j
         = 1.06066 i - 4.49027 j
         = 4.3632(cos(-75.93°) i+sin(-75.93°) j)
即: 以俯角76°速率4.36m/s著地.